Case Studies
1. Fault Investigation using electrical resistivity profiling
The recent Hayward Fault projects through a proposed housing development, and therefore the project required that a fault study be conducted. The subsurface conditions consists of a thick sequence of alluvium without surficial evidence of faulting. Regulations require that the fault be located by mapping, trenching and test borings so that houses can be offset from the actual fault traces. In order to strategically locate trenching and test borings, an electrical resistivity survey was performed to delineate changes in the lithology that may be related to displaced or truncated sedimentary beds indicative of possible faulting.
The survey consisted of six parallel profiles, each oriented perpendicular to the general trend of faulting. The profiles were spaced about 300 feet apart. All six of the profiles delineated localized subsurface discontinuities that when plotted in plan view defined a distinct lineation that was oriented to the general trend of regional faulting and corresponded with a mapped concealed section of the Hayward fault.
Two sample electrical resistivity profiles spaced 300 feet apart show a very noticeable lateral change in electrical properties where two steeply dipping zones of conductive material (blue) truncate less conductive (more resistive) alluvial material. The two zones appear to extend almost to the ground surface. These zones are interpreted as possible fault zones where fault gouge and higher permeable zones containing groundwater are the likely sources of the electrical property changes.
